I have been having problems with N mode. When I execute iwconfig get wlan0 IEEE 802.11b/g
and when I boot in Mac os my wireless card info is :

Firmware Version: Broadcom BCM43xx 1.0 5.100.198.104.5)
Supported PHY Modes: 802.11 a/b/g/n
And I am able to connect to N mode wireless network. But on ubuntu, this has been a problem for me when trying to connect to access point with phy mode N.
After some research I found on the linux wireless documentation that